Topological materials are characterised by the existence of momentum space topological defects, topological invariants and protected surface states. After the classification of the possible fully gapped topological phases, the attention has now turned to the possible momentum space topological defects appearing in gapless semimetals and nodal superconductors. In this talk I will give an overview of the gapless topological phases containing Weyl points (monopoles) and Dirac lines (vortex lines) in the momentum space. The next step in the increasing complexity of the momentum space topological defects is to consider the topology of the systems containing multiple Dirac lines. I will describe how the structural symmetries of the systems can allow stabilising a momentum-space topological defect where several Dirac lines meet and merge in the momentum space forming a momentum-space equivalent of the real space nexus considered before for Helium-3. By the bulk-boundary correspondence of topological media the presence of Dirac lines leads to formation surface states. I will discuss the properties of these surface states in a nexus semimetal.